1. Field of the Invention
The present invention relates to a method of double-sided polishing (DSP) of large format optical substrates, and, more particularly, to a method of double-sided polishing of large format optical substrates to achieve critical levels of optical performance for transmitted wave-front and beam deviation.
2. Description of Related Art
The excellent optical and physical properties of sapphire make it a strong candidate for use in optical systems that must operate in extremely demanding environments where optical transmission in the wavelength range from the visible to the mid-wave infrared is required. Accordingly, sapphire is the material of choice in a variety of current and envisioned applications that include space and military optical systems.
However, it is difficult to process sapphire by conventional fabrication processes due to its high hardness and wear resistance. A great deal of work has been done in the industry to establish useful grinding and polishing processes for small, thin sapphire wafers (typically <6″ in diameter and <0.10″ thick). But precision finishing technology for larger sapphire substrates is an especially difficult problem. Removal of residual sub-surface damage, final surface smoothness, final panel flatness, and minimization of wedge are all critical characteristics that must be met for large aerospace windows.
For example, there are current airborne optical systems that require single crystal sapphire window panels that are over 20 inches across. Significant challenges exist in the fabrication of these panels. One of the most challenging tasks involves developing high-throughput, cost-effective processing techniques which will ensure that these panels meet extremely stringent optical performance demands while retaining the mechanical strength and durability required for operation in the severe environment of high speed military jet flight profiles.
Optical fabrication involves the stepwise removal of material from an optical substrate with progressively finer grit sizes to yield a polished surface on both sides of the optical element. Conventionally, these operations are performed sequentially on a single surface at a time, first one side and then the other. In the case of high performance optical components, the part must be repeatedly “flipped” and reworked multiple times due to process-induced stress and figure distortion in order to meet the requirements for critical performance characteristics such as transmitted wave-front error (TWE). Therefore, a double-sided polishing (DSP) approach is advantageous in the fabrication of such large transparent panels. In a DSP machine, both surfaces are polished at the same time. Simultaneous removal of nearly equal amounts of material from both surfaces of the optical substrate yields a polished optic in a relatively stress-free condition in a single operation. This is particularly beneficial for large parts with high aspect ratios made from ultra-hard materials, such as sapphire, that must meet stringent TWE requirements.
However, a significant issue with double-sided polishing of large window panels has been achieving the necessary surface flatness for meeting stringent TWE and beam deviation requirements. While maintaining a low wedge is relatively simple in a DSP process, the large panel to platen size ratio and the extreme hardness of the materials such as sapphire tend to result in a bi-convex part with enough power to fail subsequent optical tests.
The prior art describes two basic applications of DSP to the polishing of sapphire. However, the prior art methodologies are for wafer-scale sapphire substrates and focus on mechanical smoothness and flatness of small diameter, thin wafers for use as substrates for GaN deposition (U.S. Pat. Nos. 5,800,725; 6,376,335; 7,214,124; 7,727,053; 8,118,646; and 8,545,712). However, the prior art is silent with respect to larger window substrates and optical performance characteristics.
“Cost effective fabrication method for large sapphire sensor windows,” by M. Walters, et al., (Proc. of SPIE Vol. 8884, SPIE, Bellingham, Wash., 2013) describes DSP of small (50 mm) disks for potential optical applications but is also silent with respect to larger substrates.
Thus, a need exists for an apparatus and method for polishing large optical substrates that have the necessary surface flatness for meeting stringent TWE and beam deviation requirements.